Poppet valves can be used to regulate the pressure of a fluid within a system by adjusting the position of a piston within a valve casing. For example, a poppet valve may be used in a de-icing system of an aircraft to control the pressure of the air off-take from a compressor stage of a gas turbine engine, the hot off-take air being used to de-ice surfaces of the aircraft.
Conventional poppet valves (e.g. bullet valves, sleeve valves etc.) contain dynamic piston seals and guide rings to provide piston sealing and sliding guidance respectively. These seals and guides operate to reduce the pressure loss across piston to casing interfaces within the poppet valve, as well as to provide smooth guiding. When a dynamic piston seal is used in a flowing fluid system, there exists a differential pressure across the dynamic piston seal which can result in a leakage flow across the seal. This leakage flow exposes components to any contaminants that are suspended within the flowing fluid. The magnitude of the leakage flow is a function of the effectiveness of the seal as well as the pressure drop across the seal.
As a result of the leakage flow, contaminants that are suspended within the flowing fluid can be drawn through a clearance between the piston and the casing and across the dynamic piston seal. In particular, contaminant particles can lodge in the seal interfaces where they may seriously affect the functioning of the seal. For example, such particles can give rise to stiction and friction which overwhelm the force margins of the piston actuation system, and thus lead to seizure of the piston in the casing. Particularly in sandy and dusty environments, pressure-regulating poppet valves can quickly become clogged with solid contaminants around the dynamic piston seals, leading to shortened operational times before seizure.
One option to avoid ingress of solid contamination to dynamic piston seals is to replace dynamic piston seals with rolling glands which are mechanically sealed and connected to both the piston and the casing. The relative movement between the piston and casing is not hampered as the gland rolls (flexes) as the piston translates. In such systems there is no flow path across the gland and hence no leakage flow. Without such a leakage flow the quantity of solid contamination drawn into the gland area is so small (even in harsh sand and dust environments) that operational performance is not affected. However, rolling glands are manufactured from elastomerics and are not suitable for use in systems operating above about 240° C.
Therefore there is a need to provide poppet valves which are operable at high temperatures and in flowing fluid conditions where the fluid may contain contaminants.